The simultaneous processing of a plurality of semiconductor wafers in a vertical batch furnace presents the problem of how to subject all wafers that are stacked into a wafer boat to substantially the same process conditions across their respective surface areas. One such process condition is the exposure to process gases. To promote the uniformity of this exposure, a vertical furnace is commonly equipped with a boat rotation mechanism that rotates the wafer boat during processing so as to average out non-uniformities in process gas flows that contact the wafers. Another process condition is the temperature of the wafers. To obtain uniform processing results across the substrates of a batch, each of the wafers thereof may preferably be heated substantially uniformly to a common temperature by heating means disposed proximate a side wall of the process chamber and proximate a top wall of the process chamber. As regards in particular the upper wafers in the wafer boat, the wafer-to-wafer temperature uniformity is generally not a significant problem, while the within-wafer temperature uniformity (due to asymmetries in the construction of the furnace) may be enhanced by the aforementioned boat rotation. However, in a vertical batch furnace the temperature of the lower substrates in the wafer boat proves difficult to control. This is partly due to the fact that they are located closely to the relatively cold lower door zone of the furnace. To mitigate the effect of their location, a pedestal supporting the wafer boat from below may be provided with additional heating means for heating the lower wafers. Although such heating means may increase the wafer-to-wafer temperature uniformity across the wafers of the batch, any non-uniformities in the heating means and/or the heat profile they produce may easily affect the within-wafer temperature uniformity of the lower wafers.
To overcome this problem, WO 2004/008491 (Dubois et al.) suggests to fit the vertical furnace with a magnetically coupled wafer rotation system for rotating the wafer boat relative to the stationary pedestal. The rotation mechanism includes a drive shaft that extends vertically inside the pedestal. The lower end of the drive shaft is magnetically coupled to a rotating motor, while the upper end, which resides in a top portion of the pedestal, is magnetically coupled to a support that is connected to the wafer boat and that itself is supported on the pedestal. The rotating motion of the motor may thus be transferred magnetically onto the (lower end of the) drive shaft, and from the (upper end of the) drive shaft onto the support of the wafer boat. In use, the boat is to be rotated relative to the pedestal so as to average out any effects the non-uniformities in the heating element may have on the temperature of the lower substrates. Remarkably, WO '491 is silent about a bearing mechanism between the pedestal and the boat. Such a bearing mechanism is understood to be an essential component of the wafer rotation mechanism. Moreover, it is a non-trivial component, in particular because the bearing mechanism would reside in the high-temperature processing environment of the furnace, which may be rich in chemical reactants that can soil and attack the bearing to shorten its life span significantly. It therefore seems that WO '491 merely discloses a speculative and non-enabling solution to the aforementioned problem.